Multi-element grating beam splitter with a reflection grating element for use in front facet substraction

ABSTRACT

A multi-element grating beam splitter with a reflection grating for use in providing a front facet detector signal in an optical system. The multi-element grating beam splitter includes first, second, third, and fourth grating elements for separating a return beam reflected and diffracted from a data track on an optical storage medium into first, second, third and fourth portions, respectively. The grating beam splitter also includes a fifth grating element, which is preferably a reflection grating, for directing a portion of an incident radiation beam onto a front facet detector. The front facet detector signal may be subtracted from a data signal resulting from detection of the first, second, third and fourth portions of the return beam in order to limit the effect of optical source noise on the data signal.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present invention is related to the following U.S. patentapplications: "Apparatus and Method for a Dual Half-Aperture FocusSensor System," Ser. No. 07/998,179, now U.S. Pat. No. 5,406,541;"Apparatus and Method for Laser Noise Cancellation in an Optical StorageSystem Using a Front Facet Monitor Signal," Ser. No. 07/961,965, nowU.S. Pat. No. 5,363,363; "Single Return Path Orthogonally-ArrangedOptical Focus and Tracking Sensor System," Ser. No. 08/259,655; and"Read/Write Laser-Detector-Grating Unit (LDGU) With AnOrthogonally-Arranged Focus and Tracking Sensor System," Ser. No.08/259,428, all assigned to the assignee of the present invention. Thedisclosures of these related Applications are incorporated herein byreference.

FIELD OF THE INVENTION

The present invention relates generally to optical read/write heads usedin optical information storage and retrieval systems. More particularly,the present invention relates to optical heads which include alaser-detector-grating unit (LDGU) and use a focus and tracking sensorsystem to control the position of a radiation beam relative to anoptical storage medium.

DESCRIPTION OF THE PRIOR ART

In many optical information storage and retrieval systems, a radiationbeam from an optical source is reflected and diffracted from a datatrack on an optical storage medium. The beam returning from the storagemedium may be directed to a detector array that provides signals used togenerate, for example, a focus error signal (FES), a tracking errorsignal (TES) and a data signal. The FES and TES generally drive servosystems for maintaining the radiation beam in-focus and on-track,respectively, relative to the storage medium. The data signal isindicative of the data stored on the data track scanned by the radiationbeam. The portion of the optical system which generates and processesthe radiation beam is generally referred to as an optical head.

The stability of an optical head is usually improved by decreasing thedistance between certain critical components, such as an optical source,beam splitter and detector array. In addition, the cost and complexityof the optical head is reduced if these components are integrated into asingle package. A known technique for accomplishing these objectivesinvolves combining components such as an optical source, a grating beamsplitter and a detector array into an integrated package generallyreferred to as a laser-detector-grating unit (LDGU). LDGUs are alsoknown as laser/detector optical heads and hologram laser units. Opticalsystems which incorporate an LDGU or a similar device will be referredto herein as LDGU-based systems. A number of exemplary LDGU-basedsystems are described in W. Ophey, "Compact Optical Light Paths," Jpn.J. Appl. Phys., Vol. 32, Part 1, No. 11B, pp. 5252-5257, November 1993.Other LDGU-based systems are described in, for example, U.S. Pat. Nos.5,050,153 and 4,945,529. An exemplary optical head in accordance withU.S. Pat. No. 4,945,529 includes a diffraction grating with four gratingregions. The four grating regions direct portions of a reflected anddiffracted radiation beam to a detector assembly in order to generate anFES, a TES and a data signal.

The above-noted LDGU-based systems suffer from a number of drawbacks.For example, the optical source is generally not sufficiently isolatedfrom the return beam, resulting in increased optical source noise. Theoptical source noise may result from phenomena such as longitudinalmode-hopping. Existing LDGUs also typically have an inherently lowthroughput efficiency, due in part to the fact that the radiation beamis generally not circularized. A circularized radiation beam isrotationally symmetrical about its optical axis. Throughput efficiencymay be defined in terms of a percentage of optical source radiationwhich is transferred to the surface of the optical storage medium.Currently available LDGUs used for reading optical disks have throughputefficiencies on the order of only about 10%, with a considerable amountof the optical source output lost in the grating beam splitter and intruncating the non-circularized radiation beam. Although LDGUs are nowcommonly used for read-only applications such as compact disk (CD)players, the problems of source noise and low throughput efficiency havelimited the usefulness of LDGUs in higher power applications such asoptical recording.

In addition, some LDGU designs exhibit excessive optical cross-talkbetween tracking and focus signals. The optical cross-talk originatesfrom, for example, diffracted radiation components and optical wavefrontaberrations in the return beam. The presence of optical cross-talk maylimit the effectiveness of LDGUs in certain optical systems,particularly those systems which utilize high performance focus andtracking servomechanisms. Although the above-cited U.S. patentapplication Ser. No. 07/998,179 reduces the effect of cross-talk inoptical heads by implementing an orthogonality condition between thefocus and tracking sensors, it does so by using separate optical pathsfor generating the focus and tracking signals. The need for additionalcomponents to create and process separate optical paths adverselyaffects the cost and complexity of the optical head.

As is apparent from the above, a need exists for an LDGU with increasedthroughput efficiency and less sensitivity to optical source noise,which is well-suited for use in optical recording applications, andwhich exhibits reduced optical cross-talk without requiring additionaloptical components.

SUMMARY OF THE INVENTION

The present invention provides an apparatus and method for generating afront facet detector signal from an incident radiation beam. The frontfacet detector signal may be used in front facet substraction in orderto limit the effect of optical source noise on, for example, a datasignal. In accordance with one aspect of the present invention, amulti-element grating beam splitter is provided for use in an opticalsystem. The optical system includes an optical source for generating aradiation beam to be applied to a data track of an optical storagemedium. The grating beam splitter includes a first grating element and asecond grating element, arranged on opposite sides of a planesubstantially parallel to a reference plane defined by an optical axisof the radiation beam and a tangent to the data track. The first andsecond grating elements separate first and second portions,respectively, of a return beam resulting from application of theradiation beam to the data track, along at least one plane substantiallyparallel to the data track. The grating beam splitter also includes athird and a fourth grating element, arranged adjacent to and on oppositesides of the first and the second grating elements, to separate a thirdand a fourth portion, respectively, of the return beam along planessubstantially perpendicular to the reference plane. The grating beamsplitter also includes a fifth grating element for directing a portionof the radiation beam to a front facet detector.

The first and second portions of the return beam may be used to generatea tracking error signal, and the third and fourth portions may be usedto generate a focus error signal. The front facet detector provides afront facet detector signal which may be subtracted from a data signalindicative of data stored on the data track to limit the effect ofoptical source noise. The optical source and the grating beam splittermay be part of an LDGU and contained within a single package.

In accordance with another aspect of the present invention, a method ofgenerating a front facet detector signal is provided. The methodincludes the steps of applying a radiation beam to a multi-elementgrating beam splitter, such that a first portion of the radiation beampasses through the multi-element grating beam splitter, and a secondportion of the radiation beam is reflected from a reflection grating inthe multi-element grating beam splitter; applying the first portion ofthe radiation beam to the data track; applying the second portion of theradiation beam to a front facet detector to generate the front facetdetector signal; separating a first and a second portion of a returnbeam resulting from application of the radiation beam to the data track,in first and second grating elements of the grating beam splitter,respectively, along at least one plane substantially parallel to areference plane defined by an optical axis of the radiation beam and atangent to the data track; and separating a third and a fourth portionof the return beam in third and fourth grating elements of the gratingbeam splitter, respectively, along planes substantially perpendicular tothe reference plane. The first and second return beam portions are usedto generate a tracking error signal, and the third and fourth portionsare used to generate a focus error signal. The front facet detectorsignal may be subtracted from, for example, a data signal resulting fromdetection of the first, second, third and fourth return beam portions.

The present invention provides an LDGU-based optical system capable ofgenerating focus error, tracking error and data signals with reducedoptical source noise and improved throughput efficiency, and withreduced cross-talk. The present invention generates these signals usinga limited number of optical components, and therefore withoutsignificantly increasing the cost and complexity of the optical system.An LDGU in accordance with the present invention may be used to bothread from and write to an optical recording medium.

Further features of the invention, its nature and various advantageswill become more apparent from the accompanying drawings and thefollowing detailed description of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side-sectional view of an optical system which includes anexemplary LDGU in accordance with the present invention.

FIG. 2 is a view of the exemplary LDGU of FIG. 1 taken along the sectionline 2--2.

FIG. 3 is a detailed view of an exemplary blazed grating beam splitterin accordance with the present invention.

FIG. 4 is a detailed view of an exemplary detector array in accordancewith the present invention.

FIG. 5 is a side-section view of an optical system which includes analternative embodiment of an exemplary LDGU in accordance with thepresent invention.

FIG. 6 is a view of the exemplary LDGU of FIG. 5 taken along the sectionline 6--6.

FIG. 7 is a side-sectional view of an optical system which includesanother alternative embodiment of an exemplary LDGU in accordance withthe present invention.

FIG. 8 is a detailed view of an exemplary blazed grating beam splittersuitable for use in the LDGU of FIG. 7.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows an exemplary optical system 20 in accordance with thepresent invention. The components of system 20 which process, direct anddetect the return beam to provide the FES and TES, and in some cases adata signal, may be collectively referred to as a focus and trackingsensor system. The system 20 may also include additional opticalcomponents, such as reflectors, lenses, beam splitters, quarter-waveplates, and gratings, for directing the incident and return radiationbeams in directions other than those shown. Furthermore, although thepresent invention is particularly well-suited for use in opticalread/write heads, it may also provide advantages in other opticalapplications, including, for example, position sensors.

The system 20 includes an LDGU 30 which may be used in an opticalread/write head to both read from and write to optical storage mediasuch as recordable CDs. The LDGU 30 combines several optical componentsinto a single package. The LDGU 30 includes a package housing 32, atransparent substrate 34 and a package base 36. The transparentsubstrate 44 may be glass, plastic, or other transparent material.Although the package shown is a can type package, the various componentsof LDGU 30 may be enclosed in other types of packages as required for agiven application. A number of contact pins 38 protrude from the packagebase 36 for connecting the LDGU 30 to external electronic circuitry (notshown). The LDGU 30 also includes an optical source 40 which istypically a laser diode. Alternatively, the optical source 40 may be acompact laser. The optical source 40 generates a radiation beam which isincident on a grating beam splitter 42 formed on an inner surface of thetransparent substrate 34. The grating beam splitter 42 is preferably ablazed grating beam splitter.

The transparent substrate 34 is arranged between the optical source 40and an optical storage medium such that the radiation beam passesthrough the substrate. A zeroth order diffraction component of theradiation beam passes undeflected through the transparent substrate 34and the grating beam splitter 42 formed thereon and is collimated bycollimating lens 44. The radiation beam is then focused by an objectivelens 52 onto an optical storage medium 56, which may be, for example, arecordable CD. Only a portion of the optical storage medium 56 is shownin FIG. 1. The radiation beam is used to store and retrieve informationfrom the optical storage medium 56, and typically has a linearpolarization. Alternatively, the incident radiation beam could haveother polarizations. For example, a quarter-wave plate (not shown) maybe arranged between collimating lens 44 and objective lens 52 to providea circular polarization to the radiation beam. In other embodiments, theincident radiation beam could be unpolarized.

Any of a number of well-known techniques may be used to form the blazedgrating beam splitter 42 on the transparent substrate 34. For example,appropriate grating patterns could be photolithographically formed in alayer of photoresist on a surface of substrate 34, an ion milling beamcould be used to mill the grating patterns onto the substrate 34, or thegrating patterns could be formed using molded clear epoxy or resins. Inaddition, the grating beam splitter could be formed using holographictechniques, in which, for example, two or more laser beams are used tocreate an interference pattern in a thin layer of photoresist. These andother grating formation techniques are well-known in the art and willnot be further described herein. Furthermore, although the grating beamsplitter 42 is shown in LDGU 30 on an inner surface of transparentsubstrate 34, it could also be formed on an outer surface of thesubstrate, or partially formed on both inner and outer surfaces of thesubstrate. It may be preferable in many applications, however, to formthe grating beam splitter 42 on an inner surface in order to protect itfrom contaminants.

In another embodiment of the present invention, the transparentsubstrate could be, for example, a thin film on which a grating beamsplitter is formed. The thin film could be mounted in an aperture (notshown) in LDGU 30 such that the incident radiation beam and return beampass through the thin film transparent substrate and the grating beamsplitter. In these and other embodiments, the grating beam splitter mayalternatively be formed within the transparent substrate, rather than onan inner or outer surface thereof. The term "transparent substrate" isdefined herein as any transparent material, including glass, plastic orfilm, which may be used to support a grating beam splitter formedtherein or thereon.

The optical storage medium 56 includes a surface 56A having a number ofdata tracks formed thereon. Each data track 56B is shown incross-section and generally extends in a direction perpendicular to theplane of the drawing. The data track 56B is a type of diffractioncomponent-generating structure. The structure diffracts the incidentradiation beam because the depth of the structure is generally afraction of the wavelength of the incident radiation beam and introducesphase differences in the return beam. Although the data track 56B isshown as a raised structure in the exemplary system 20 of FIG. 1, a datatrack in accordance with the present invention may also be, for example,a groove in the storage medium, a region between two grooves in thestorage medium, a series of unconnected raised regions, or other opticalpath structures of appropriate dimension and refractive index such thatdiffraction patterns are created in response to an incident radiationbeam.

It should be noted that although the data tracks are generally arrangedin a spiral configuration on an optical storage medium such as arecordable CD, a given portion of the data track 56B around a pointcurrently illuminated by the incident radiation beam will exhibit littlecurvature and therefore such a portion may be considered substantiallystraight. A projection of such a portion of data track 56B on thegrating beam splitter 42 will generally lie in a plane separating partof the grating beam splitter 42 into first and second grating elements.A dashed line 58 is drawn in FIG. 1 between a currently illuminated datatrack 56B and the center of the grating beam splitter 42. The line 58represents an optical axis of the incident radiation beam and isperpendicular to the data track 56B and the projection thereof ontograting beam splitter 42. A reference plane is defined herein by theoptical axis 58 of the incident radiation beam and a tangent to the datatrack 56B at the point currently illuminated by the incident radiationbeam. The substantially straight portion of the data track 56B may beconsidered part of the tangent to the data track 56B. The projection ofdata track 56B onto grating beam splitter 42 also generally lies withinthe reference plane.

The data track 56B reflects and diffracts the incident radiation beamapplied thereto. The reflected and diffracted incident radiation beamwill be referred to herein as a return beam. The return beam is thenincident on the grating beam splitter 42, which separates the returnbeam into a number of different portions. These portions are directedtowards a detector array 68. The detector array 68 detects the variousportions of the return beam and generates signals which, when combinedin the manner described below, provide a TES, an FES and a data signal.

A portion of the incident radiation beam from optical source 40 passingthrough the grating beam splitter 42 is diffracted by the grating beamsplitter 42 toward a reflector 64. The reflector 64 is mounted in andsupported by a cap 65. Alternatively, the reflector 64 could beincorporated into the transparent substrate 34 or placed elsewhere inthe LDGU package. The reflector 64 reflects the diffracted portion ofthe incident radiation beam back through the grating beam splitter 42 toa front facet detector 72. The incident radiation beam from opticalsource 40 is thus separated by grating beam splitter 42 into tworadiation beam portions. The first radiation beam portion is applied tothe data track 56B of the optical storage medium 56. The secondradiation beam portion, which includes light diffracted from gratingelements A, B, C and D, is reflected by reflector 64 and applied tofront facet detector 72. The front facet detector is an opticaldetector, such as a photodiode, which generates a signal indicative ofthe optical power level of the incident radiation beam supplied fromsource 40. The front facet detector signal is used herein to limit theeffect of optical source noise. An exemplary technique for using thefront facet detector signal to control optical source noise is referredto as front facet subtraction and is described in the above-cited U.S.patent application Ser. No. 07/961,965. Other noise reduction techniquesbased on a signal indicative of optical source power may also be used.

Front facet subtraction reduces the effect of optical source noise in adetected return beam. Many commonly-used optical sources have a numberof different lasing modes, each producing a radiation beam at a slightlydifferent wavelength. Part of the return beam reflected from the opticalstorage medium returns to the optical source and may producelongitudinal mode-hopping, in which the source hops between two or moreof its lasing modes. Longitudinal mode-hopping generally causesintensity noise on the radiation beam produced by the source. Frontfacet subtraction involves detecting a portion of the incident radiationbeam before it arrives at the optical storage medium, adjusting itsamplitude and phase delay, and subtracting it from the detected returnbeam. The technique is referred to as front facet subtraction becausethe incident radiation beam, a portion of which is subtracted from thedetected return beam, generally exits the front facet of a laser diodeoptical source. Front facet subtraction may be applied in the presentinvention by, for example, subtracting a signal generated by the frontfacet detector 72 from signals generated by the detectors in detectorarray 68. In one possible embodiment of the present invention, a frontfacet detector signal is subtracted from the data signal describedbelow. Additional detail may be found in the above-cited U.S. patentapplication Ser. No. 07/961,965.

FIG. 2 shows a view of the LDGU 30 taken along the section line 2--2 ofFIG. 1. A post 71 holds the optical source 40 in place and also servesas a heat sink. The front facet detector 72 and detector array 68 aresuitably arranged for receiving portions of the incident and returnbeams, respectively, as described above. The exemplary LDGU 30 alsoincludes a preamplifier 76 connected to the detector array 68 foramplifying signals generated in the detector array 68 and/or the frontfacet detector 72. The amplified signals from preamplifier 76 are thensupplied to electronic circuitry (not shown) for combining the signalsto generate an FES, a TES and a data signal in a manner to be describedbelow. Alternatively, certain detector signals, such as those generatedby detector elements c, c', d and d', could be combined prior toamplification. The preamplifier 76 preferably includes a separatelow-noise amplifier for each detector in the detector array 68 and maybe implemented as, for example, an Application-Specific IntegratedCircuit (ASIC). Exemplary types of preamplifiers which may be usedinclude transimpendance amplifiers. The placement of preamplifier 76within LDGU 30 allows for short lead lines between a given detector andits corresponding low-noise amplifier, and therefore reduces noisepickup and allows an increase in signal bandwidth. The electricalinterconnections between the low-noise amplifiers in preamplifier 76 andthe detectors in detector array 68 would be readily apparent to oneskilled in the art and are therefore not shown. In alternativeembodiments, the preamplifier 76 could be eliminated.

FIG. 3 shows a detailed view, in a plane parallel to the plane ofsection 2--2, of an exemplary blazed grating beam splitter 42 inaccordance with the present invention. The exemplary blazed grating beamsplitter 42 includes first, second, third and fourth grating elements A,B, C and D, respectively. In a preferred embodiment of the presentinvention, the grating elements A, B, C and D of the grating beamsplitter 42 are blazed gratings. Blazed gratings are commonly used inoptical systems and their operation and high efficiency properties aregenerally well-known. In alternative embodiments, other types ofgratings could be used, including, for example, sinusoidal gratings,ruled gratings and holographic structures. Each grating element includesa grating pattern as shown in FIG. 3. The line spacings, line widths,blaze angles, and other dimensions of the grating patterns in eachgrating element may vary depending upon the application, and can bereadily determined in a well-known manner.

The first and second grating elements A and B of FIG. 3 are dividedalong a line 82 which is parallel to the above-described tangent to thedata track 56B. The line 82 is also substantially parallel to aprojection 83 of the tangent to the data track 56B onto the grating beamsplitter 42, and lies in the reference plane defined by the optical axis58 and the tangent to the data track 56B. The third and fourth gratingelements C and D are arranged adjacent to and on opposite sides of thefirst and second grating elements A and B. The elements C and D areseparated from elements A and B by lines 84 and 86, respectively, whichare perpendicular to the data track 56B or the projection 83 of the datatrack 56B on the grating beam splitter 42. The grating elements A, B, Cand D separate the return beam into four different portions, alongplanes which contain the lines 82, 84 and 86. The first and secondgrating elements A and B separate the return beam along the referenceplane defined above. In other embodiments, the first and second gratingelements could separate the return beam along another planesubstantially parallel to the reference plane, or along two differentplanes substantially parallel to the reference plane. The third andfourth grating elements C and D separate the return beam along planessubstantially perpendicular to the reference plane. In this exemplaryembodiment, each of the resulting portions of the return beam is focusedon a different detector in detector array 68.

FIG. 4 shows the exemplary detector array 68 in greater detail. Thedetector array 68 includes four detectors a, b, c and d, d' fordetecting the first, second, third and fourth portions of the returnbeam, respectively. The first, second and third detectors are singleelement detectors, designated in FIG. 4 as detector elements a, b and c,respectively. The fourth detector is a dual element detector withdetector elements d and d'. In other embodiments, the third detector cmay be a dual element detector rather than a single element detector.Each detector element may be, for example, a photodiode, a group ofphotodiodes, or another type of photodetector. Exemplary focus spots 92,94, 96 and 98 indicate an area of each detector on which the first,second, third and fourth return beam portions, respectively, may befocused when the incident radiation beam is on-track and in-focusrelative to the optical storage medium. It should be emphasized thatthis particular arrangement of detectors is exemplary only. For example,the detectors shown may include additional detector elements or fewerdetector elements in other embodiments of the present invention. Inaddition, each of the detectors need not be part of a single detectorarray. As will be discussed in greater detail below, the gratingelements and corresponding detector elements are arranged such that theoptical cross-talk between tracking and focus signals is minimized.

In the exemplary embodiment of FIG. 3, the grating patters shown aresuitable for directing the first, second, third and fourth separatedportions of the return beam onto detectors a, b, c and d, d',respectively, of detector array 68. The pattern line spacings in elementC are wider than those in D, and in both C and D the pattern lines aresubstantially parallel to line 82. The pattern line spacings in elementsA and B are the same, while narrower than those in C and wider thanthose in D. In addition, the pattern lines in A and B are slanted atequal but opposite angles relative to line 82.

It should also be noted that the arrangement of grating elements shownin FIG. 3 is exemplary only, and alternative embodiments of the presentinvention may utilize other arrangements. For example, the variouselements of the grating beam splitter 42 may be separated by lines whichdeviate from the parallel or perpendicular lines shown in FIG. 3 by upto about ten percent. The terms "substantially parallel" and"substantially perpendicular," as used herein, include deviations of atleast ten percent from parallel and perpendicular, respectively.Although the amount of optical cross-talk may increase as a result ofsuch deviations, an improvement over most current prior art systemswould generally still be obtained. In addition, although a four elementgrating beam splitter may be preferred in many applications, the gratingbeam splitter could include more or less than four grating elements. Forexample, an embodiment which does not require a data signal may includeonly the three grating elements A, B and C, or A, B and D. In athree-element embodiment, the return beam is separated into threeportions, along a subset of the planes described above in conjunctionwith FIG. 3, and the third portion of the return beam is used togenerate an FES. The third detector could be a dual element detector c,c', similar to detector d, d', suitable for use in generating an FES. Ina three-element embodiment which does require a data signal, theelements A and B could be modified in size and shape to also receive theportion of the return beam which would otherwise fall on the removedelement, such that the entire return beam is still incident on thedetector array. The grating beam splitter 42 may also include additionalelements other than the elements shown. One skilled in the art couldreadily modify the embodiments of FIGS. 1, 3 and 4 to accommodate suchadditional elements.

In general, the return beam includes a reflected component, alsoreferred to as a zeroth order diffraction component, and a number ofhigher order diffraction components diffracted by the surface of theoptical storage medium. A given diffraction order generally includesboth a positive and a negative diffraction component. Although higherorder diffraction components may also be present in the return beam, thepresent invention can be readily understood without furtherconsideration of diffraction components greater than first order. Whenthe reflected component overlaps with the first order diffractedcomponents, interference occurs. This interference may be directed todetectors a and b to provide, for example, a push-pull TES, as will bedescribed below. The two first order diffraction components may be, forexample, contiguous with an optical axis of the incident radiation beam,and therefore both may overlap with the reflected component. It shouldbe noted, however, that the present invention may be utilized in systemsin which the positive and negative diffraction components overlap witheach other as well as with the reflected components. Additional detailregarding diffraction components may be found in, for example, theabove-cited U.S. patent application Ser. No. 07/998,179, and pp. 172-179of A. Marchant, "Optical Recording: A Technical Overview,"Addison-Wesley, Reading, Mass., which are incorporated by referenceherein.

A tracking error signal (TES) may be generated from the first and secondportions of the return beam incident on the first and second detectors aand b, respectively, of the detector array 68. The TES is generated inaccordance with the relationship a-b, which indicates that the signalgenerated by detector element b is subtracted from the signal generatedby detector element a. As a result of passing through theabove-described grating beam splitter 42, the first and second portionsof the return beam each may include a different diffraction component ofa given diffraction order, diffracted from the optical storage medium,as well as undiffracted components. The different diffraction componentmay be either a positive or a negative diffraction component. It shouldbe understood that, in general, only part of any given diffractioncomponent, rather than the entire component, falls within the objectivelens aperture and will therefore be incident on grating beam splitter42. References made herein to a particular diffraction component arethus meant to include any part of that component.

A focus error signal (FES) may be generated from the fourth portion ofthe return beam incident on the fourth detector d, d' of the detectorarray 68. An FES is generated in accordance with the relationship d--d',which indicates that the signal generated by detector element d' issubtracted from the signal generated by detector element d. As a resultof passing through the above-described grating beam splitter 42, thethird and fourth portions of the return beam include both positive andnegative diffraction components of a given diffraction order, diffractedfrom the optical storage medium, as well as undiffracted components.Each of the detector elements d and d' thus receive both diffractioncomponents of a given diffraction order. By subtracting the signalsresulting from detection of the fourth portion of the return beam ondetector elements d and d', the diffraction components of a givendiffraction order substantially cancel out, thereby reducing opticalcross-talk. In other embodiments, a dual element detector c, c' could beused for detector c in array 68, with the detector elements c and c'arranged in the same manner as detector elements d and d' of FIG. 4. Thethird portion of the return beam, alone or in combination with thefourth portion, could then be used to generate an FES. For example, anFES could be generated in accordance with the relationship c--c', whichindicates that the signal generated by detector element c' is subtractedfrom the signal generated by detector element c. Alternatively, an FEScould be generated in accordance with the relationship (c+d')-(c'+d),which indicates that the sum of the signals generated by detectorelements c' and d is subtracted from the sum of the signals generated bythe detector elements c and d'. In such an embodiment, signals generatedby diagonally-arranged detector elements are summed, and the resultingsums subtracted, to provide the FES.

A data signal, indicative of the data stored on data track 56B, may alsobe generated in the optical system 20. For example, a data signal couldbe generated by combining the signals generated by each detector elementin the detector array 68, in accordance with the relationshipa+b+c+d+d'. Alternatively, signals from a subset of detector elementscould be combined to generate a data signal.

System 20 may also include electronic circuitry (not shown) forcombining signals generated by the detector elements of detector array68. The electronic circuitry may include adders, subtracters or othertypes of signal combiners for generating focus error, tracking error anddata signals in accordance with the above-described relationships. Suchelectronic circuitry is generally well-known in the art and willtherefore not be further described herein.

In general, the orientation and location of the detector elements a andb is not critical to the operation of the present invention, and thearrangement in FIG. 4 or other alternative arrangements may be chosen inorder to satisfy detector array packaging constraints or other criteria.The position of the fourth detector elements d and d' may also be variedbut the division between the pair should generally be along a linesubstantially perpendicular to the data track 56B, or the projection 83of the data track 56B on the grating beam splitter 42. This divisionline is also substantially perpendicular to the above-defined referenceplane.

The grating beam splitter 42 of the present invention may be replacedwith other optical devices capable of dividing the return beam reflectedand diffracted from a data track into a number of distinct portions inaccordance with the above description. Alternatives to the grating beamsplitter 42 include, for example, holograms. In addition, as mentionedabove, the grating or other optical device used to separate the returnbeam into its respective portions may include more or less than fourelements. The elements could be suitably arranged to separate the returnbeam into portions which, when detected, generate signals which may becombined in accordance with the present invention such that opticalcross-talk is minimized.

FIG. 5 shows an alternative embodiment of the present invention in whicha lens 100 is arranged between the optical source 40 and the gratingbeam splitter 42 in the path of the radiation beam. The lens 100 isaligned and mounted in front of the optical source and substantiallycircularizes the radiation beam. A radiation beam from an optical sourcesuch as a laser diode generally has an elliptical cross-section. Whensuch a radiation beam is circularized, it becomes rotationally symmetricabout its optical axis, and exhibits a generally circular cross-section.A circularized radiation beam in accordance with the present inventionneed not be perfectly circular in cross-section. The lens 100significantly improves the optical throughput efficiency of the LDGU inpart because less light must be truncated from the radiation beam inorder to produce a reasonably round focused spot at the optical mediumsurface. The lens 100 is preferably an anamorphic lens and may be, forexample, a model VPS700 refractive lens available from Blue SkyResearch, San Jose, Calif. Alternatively, the lens 100 may be adiffractive lens rather than a refractive lens. FIG. 6 shows a view ofthe LDGU 30 of FIG. 5 taken along the section line 6--6. A base 102, orother suitable mechanism, is attached to lens 100 and supports the lens100 in front of the optical source 40. A bracket 104 supports base 102.In alternative embodiments, the lens 100 could be placed at any of anumber of other locations in the path of the incident radiation beam.

In another embodiment of the present invention, the transparentsubstrate 34 and grating beam splitter 42 could be arranged between thecollimating lens 44 and the objective lens 52. The grating beam splitter42 would generally be located close to the collimating lens 44 in suchan embodiment. The location and design of the reflector 64 could then besuitably modified to reflect the diffracted portions of the incidentradiation beam to the front facet detector 72. This alternativeembodiment could be used in the systems of FIGS. 1, 5 and 7.

In another embodiment of the present invention, the LDGU 30 might alsoincorporate all or a portion of the optical source driver electronics.The driver electronics may include, for example, a high frequencyinjection (HFI) circuit and a laser driver, both of which are well-knownin the art. HFI is described in, for example, E. Gage and S. Beckens,"Effects of high frequency injection and optical feedback onsemiconductor laser performance," SPIE Proceedings, Vol. 1316, OpticalData Storage, pp. 199-204, March 1990, which is incorporated byreference herein. In addition, the LDGU 30 could include a rear facetdetector. The rear facet detector is typically an optical detector, suchas a photodiode, which detects light radiated out a back end of anoptical source, also referred to as a rear facet in the context of alaser diode optical source. The above-described front facet subtractiontechnique could be implemented using a rear facet detector incombination with or in place of the front facet detector. However, theuse of a front facet detector is generally preferred in that lightradiated from a rear facet may not be perfectly correlated in intensityto the light radiated from the front facet.

FIG. 7 shows optical system 20 with another alternative embodiment of anLDGU in accordance with the present invention. In the embodiment shown,reflector 64 and cap 65 are eliminated, and four-element grating beamsplitter 42 is replaced with a five-element grating beam splitter 142.FIG. 8 shows a detailed view, in a plane parallel to the plane ofsection 2--2, of grating beam splitter 142. Grating beam splitter 142includes grating elements A, B, C and D, arranged as described above inconjunction with FIG. 3. Grating beam splitter 142 also includes a fifthgrating element E, circular in shape, arranged between grating elementsA, B, C and D. The fifth grating element E directs a portion of theincident radiation beam, shown in FIG. 7 as a beam 157, onto the frontfacet detector 72. The grating element E is preferably a reflectiongrating, which can generate a number of reflected diffraction orders.The beam 157 is thus referred to herein as a reflected diffracted beam,and is detected in front facet detector 72. The resulting front facetdetector signal may be used, for example, to perform front facetsubtraction in the manner previously described.

The grating elements A, B, C and D of grating beam splitter 142 separatea return beam into first, second, third and fourth portions,respectively, along planes containing lines 82, 84 and 86. Gratingelement E is also preferably a blazed grating, and in the exemplaryembodiment shown is centered on the optical axis of the radiation beam,on line 82 adjacent elements A and B and midway between elements C andD. In order to direct the beam 157 to the front facet detector 72,element E includes grating pattern lines oriented substantially parallelto the grating pattern lines of elements C and D, but with line spacingsnarrower than those in element D. Of course, alternative gratingpatterns could also be used to direct the beam 157. The efficiency ofthe grating element E may be improved by coating it with a reflectivematerial, such as gold or aluminum. The grating element E could beopaque, such that it generates only reflected diffraction orders, or itcould be partially transparent, such that it generates both transmittedand reflected diffracted orders.

It should be noted that the size, shape and location of element E areexemplary only. The size and shape of element E will generally dependupon the amount of light needed to generate the front facet detectorsignal, the efficiency of the grating, and the location of the element.In addition, the element E could be located in any of a number of otherpositions in the grating beam splitter 142, as long as it is able toreceive and diffract a portion of the incident radiation beam.Furthermore, the element E may be replaced with two or more discretereflection grating elements, placed in various locations in the gratingbeam splitter 142.

The embodiment of FIGS. 7 and 8 may provide advantages in certainapplications. In general, optical head alignment is simplified becausethe beam directed to the front facet detector 72 includes lightdiffracted from a single grating element, rather than light diffractedfrom each of the four grating elements A, B, C and D as in theembodiment of FIGS. 1 and 3. A single alignment of the grating beamsplitter 142 can provide proper detection of the front facet detectorsignal generated from the incident radiation beam, as well as the focusand tracking error signals generated from the return beam. Furthermore,the reflected diffraction orders 157 provided by element E does notrequire, and additional converging means such as reflector 64 and cap 65is not needed.

Although the foregoing detailed description has illustrated the presentinvention primarily in terms of a particular optical information storageand retrieval system, it should be understood that the embodimentsdescribed are exemplary only. Many variations may be made in thearrangements shown, including, for example, the type of grating beamsplitter used to separate the return beam and the arrangement, shape andnumber of grating elements, the number of portions into which the returnbeam is separated, the arrangement of detectors and detector elementsonto which the portions of the return beam are focused, and the type andarrangement of optical components for directing the incident and returnradiation beams in the optical system. These and other alternatives andvariations will be readily apparent to those skilled in the art, and thepresent invention is therefore limited only by the appended claims.

    ______________________________________    PARTS LIST    ______________________________________    A, B, C, D, E grating elements    a, b, c, d, d'                  detectors    20            optical system    30            laser-detector-grating unit (LDGU)    32            package housing    34            transparent substrate    36            package base    38            contact pins    40            optical source    42            grating beam splitter    44            collimating lens    52            objective lens    56            optical storage medium    56A           optical storage medium surface    56B           data track    58            optical axis    64            reflector    65            cap    68            detector array    71            post    72            front facet detector    76            preamplifier    82            line    83            projection    84, 86        lines    92, 94, 96, 98                  focus spots    100           lens    102           base    104           bracket    142           grating beam splitter    157           reflected diffracted beam    ______________________________________

What is claimed is:
 1. A multi-element grating beam splitter for use inan optical system having an optical source for generating a radiationbeam to be applied to a data track of an optical storage medium, saidgrating beam splitter comprising:a first grating element; a secondgrating element, said first and said second grating elements arranged onopposite sides of a plane substantially parallel to a reference planedefined by an optical axis of said radiation beam and a tangent to saiddata track, such that said first and said second grating elementsseparate a first and a second portion, respectively, of a return beamresulting from application of said radiation beam to said data track,along at least one plane substantially parallel to said data track; athird grating element, arranged adjacent to and on one side of saidfirst and said second grating elements, to separate a third portion ofsaid return beam along a plane substantially perpendicular to saidreference plane; a fourth grating element, arranged adjacent to and onan opposite side of said first and said second grating elements, toseparate a fourth portion of said return beam along another planesubstantially perpendicular to said reference plane; and a fifth gratingelement for directing a portion of said radiation beam to a front facetdetector.
 2. The grating beam splitter of claim 1 wherein said first andsaid second portions of said return beam provide a tracking errorsignal.
 3. The grating beam splitter of claim 1 wherein said third andsaid fourth portions of said return beam provide a focus error signal.4. The grating beam splitter of claim 1 wherein said portion of saidradiation beam directed to said front facet detector provides a frontfacet detector signal suitable for subtraction from a data signalindicative of data stored on said data track.
 5. The grating beamsplitter of claim 1 wherein said fifth grating element is circular inshape.
 6. The grating beam splitter of claim 1 wherein said fifthgrating element is a reflection grating.
 7. The grating beam splitter ofclaim 1 wherein said fifth grating element is coated with a reflectivematerial.
 8. The grating beam splitter of claim 1 wherein said fifthgrating element is opaque.
 9. The grating beam splitter of claim 1wherein said fifth grating element is partially transparent.
 10. Thegrating beam splitter of claim 1 wherein said fifth grating element isarranged between said first, second, third and fourth grating elements.11. The grating beam splitter of claim 10 wherein said fifth gratingelement is centered between said third and said fourth grating elementson a line separating said first and said second grating elements.
 12. Amethod of providing a front facet detector signal in an optical system,said optical system having an optical source for generating a radiationbeam to be applied to a data track of an optical storage medium, saidmethod comprising the steps of:applying said radiation beam to amulti-element grating beam splitter, such that a first portion of saidradiation beam passes through said multi-element grating beam splitter,and a second portion of said radiation beam is reflected from areflection grating in said multi-element grating beam splitter; applyingsaid first portion of said radiation beam to said data track; applyingsaid second portion of said radiation beam to a front facet detector togenerate said front facet detector signal; separating a first and asecond portion of a return beam resulting from application of saidradiation beam to said data track, in first and second grating elementsof said grating beam splitter, respectively, along at least one planesubstantially parallel to a reference plane defined by an optical axisof said radiation beam and a tangent to said data track; and separatinga third and a fourth portion of said return beam in third and fourthgrating elements of said grating beam splitter, respectively, alongplanes substantially perpendicular to said reference plane.
 13. Themethod of claim 12 further including the step of generating a trackingerror signal from said first and said second portions of said returnbeam.
 14. The method of claim 12 further including the step ofgenerating a focus error signal from said third and said fourth portionsof said return beam.
 15. The method of claim 12 further including thestep of subtracting said front facet detector signal from a data signalindicative of data stored on said data track.
 16. The method of claim 12further including the steps of:detecting said first, second, third andfourth portions of said return beam in first, second, third and fourthdetectors of a detector array, respectively; generating a data signalindicative of data stored on said data track by combining signals fromsaid first, second, third and fourth detectors; and subtracting saidfront facet detector signal from said data signal.